In the Liverworts the young sporogonium lives like a parasite, being nourished by the sexual generation (only in Anthoceros has it a slight power of assimilation). In the Leafy-Mosses, on the other hand, with regard to the power of assimilation, all transitions are found from abundant assimilation (Funaria, Physcomitrium) to almost complete “parasitism” (Sphagnum, Andreæa). In the majority of the operculate Mosses the sporogonium has a more or less perfect system of assimilation, and is able itself to form a large portion of the material necessary for the development of the spores, so that it chiefly receives from the sexual generation the inorganic substances which must be obtained from the soil. The more highly developed the assimilative system of the sporogonium, the more stomata are present.
Apospory. In some operculate Mosses it has been possible to obtain a protonema with small Moss-plants from the seta, when severed from its Moss-plant, and grown on damp sand.
The Mosses are the lowest plants which are provided with stem and leaf. They are assigned a lower place when compared with the higher Cryptogams, partly because there are still found within the Division so many forms with a mere thallus, partly because typical roots are wanting and the anatomical structure is so extremely simple, and partly also because of the relation between the two generations. The highest Mosses terminate the Division, the Muscineæ and Pteridophyta having had a common origin in the Algæ-like Thallophyta.
They are divided into two classes:—
Hepaticæ, or Liverworts.
Musci frondosi. True Mosses or Leafy-Mosses.
Class 1. Hepaticæ (Liverworts).
The protonema is only slightly developed. The remaining part of the vegetative body is either a prostrate, often dichotomously-branched thallus, pressed to the substratum (thalloid Liverworts), with or without scales on the under side (Figs. [194], [197]); or a thin, prostrate, creeping stem, with distinctly-developed leaves, which are borne in two or three rows (Figs. [195], [198]), viz., two on the upper and, in most cases, one on the under side. The leaves situated on the ventral side (amphigastria) are differently shaped from the others (Fig. [198] a), and are sometimes entirely absent. In contradistinction to the Leafy-Mosses, stress must be laid on the well-marked dorsiventrality of the vegetative organs; i.e. the very distinct contrast between the dorsal side exposed to the light and the ventral side turned to the ground. Veins are never found in the leaves.
The ventral part of the archegonium (calyptra) continues to grow for some time, and encloses the growing embryo, but when the spores are ripe it is finally ruptured by the sporangium, and remains situated like a sheath (vaginula) around its base. The sporangium opens, longitudinally, by valves or teeth (Fig. [194], [195], [197] b), very rarely by a lid, or sometimes not at all. A columella is wanting (except in Anthoceros, Fig. [194]); but on the other hand, a few of the cells lying between the spores are developed into elaters (Fig. [196]), i.e. spindle-shaped cells with spirally-twisted thickenings, which are hygroscopic, and thus serve to distribute the spores. (They are seen in Fig. [189] C, not yet fully developed, as long cells radiating from the base of the sporangium. They are wanting in Riccia).
